The present exemplary embodiments relate to the synthesis of polyaniline and its substituted derivatives. It finds particular application in conjunction with the synthesis of aligned electrically conductive and non-conductive polyaniline nanofibers, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Electroactive conductive polymers have been subject to extensive research in recent years. Polymers which show electrical conductivity due to the structure of the polymeric chain may be used to replace metal conductors and semiconductor materials in many applications. Significant applications include providing a conductive pathway in circuits and devices, displays, lighting, chemical, biological, environmental and medical sensors, anticorrosive coatings, scaffolds for tissue growth, antistatic shielding (ESD) and electromagnetic shielding (EMI).
In the group of intrinsically electrically conductive polymers, one technically promising polymer is polyaniline. Polyaniline has emerged as one of the most promising conducting polymers and can be used in a variety of applications, such as paint, antistatic protection, electromagnetic radiation protection, electro-optic devices such as liquid crystal devices (LCDs), light emissive displays, lighting and photocells, transducers, circuit boards, chemical, biological, environmental and medical sensors, anticorrosive coatings, scaffolds for tissue growth, etc.
Polyaniline is one of a class of conductive polymers, which can be synthesized through either chemical polymerization or electrochemical polymerization. Polyaniline is conventionally prepared by polymerizing an aniline monomer. The nitrogen atoms of monomer units are bonded to the para-carbon in the benzene ring of the next monomer unit. In chemical preparation, bulk polymerization is the most common method to make polyaniline. As has been previously reported, conventional bulk chemical synthesis produces granular polyaniline.
Thus, a variety of other chemical methods have been used in order to obtain polyaniline nanofibers. These approaches include use of templates or surfactants, electrospinning, coagulating media, interfacial polymerization, seeding, and oligomer-assisted polymerization. Among these methods, interfacial polymerization is perhaps the easiest and least expensive means to obtain nanofibers in one step. However, this method requires organic solvents to dissolve the aniline monomer, resulting in a waste stream that must be treated.
It is known that a thin film of polyaniline is deposited on substrates in the conventional polymerization of aniline, called in-situ adsorption polymerization. However, the deposited thin film prepared by the conventional chemical polymerization is only composed of irregular granular particulates. Until now, aligned and oriented nanostructures of polyaniline are generally produced in chemical or electrochemical polymerization through the assistance by hard templates. In the hard-template polymerization, polyaniline is confined to growth inside the channels of the membranes. After polymerization, the templates have to be removed or etched away carefully to obtain an oriented nanostructured thin film. The diameters of the aligned nanofibers are limited by the sizes of the pores of the template membranes used. Recently, a step-wise electrochemical deposition process was introduced to deposit oriented polyaniline nanofibers on the conductive substrates (e.g. Au, Pt etc.) without using a hard template. This method is limited by the substrates. It is necessary to use electrically conductive materials for substrates to obtain oriented nanostructures. For template or step-wise electrochemical deposition methods, a very large uniform array of aligned and oriented nanostructures is generally not possible to fabricate (e.g. a letter sized substrate, 8.5 inches×11 inches). Those limitations restrict the applicability of the aligned nanofibers, especially for use in surface response (e.g., superhydrophobic or superhydrophilic surfaces), electrodes for organic or polymeric light-emitting diodes, field emission display, DNA stretching, chemical sensors, biosensors etc.
Therefore, there is an interest in devising an easy, inexpensive, environmentally friendly and scalable one-step method to produce highly pure, uniform nanofibers with controllable average diameters ranging from 5 nm to 250 nm in bulk quantities to meet the requirements for potential use on cell culture, electronic devices, sensors, biosensors, supercapacitors, hydrogen storage, etc.